Also we can look at previous game gens here.
Gen 4 had Dialga and Palkia in Plat with Giratinas origin form
Im not going to metion HGold/SSilver, they had both legendaries in the anyway,just at different point.
Emerald Had G and K in it.
Maybe you can choose who you want like in HG/SS's Palikia Dialga Giratina event.Or Latios and Latias im Emerald.
I like that idea,im going with that =)